European Published Patent Application No. 0 712 683 describes a machine tool that is suitable for the 5-axis machining of workpieces and includes a stationary machine frame as well as a tool head, which can be positioned along three mutually orthogonal translation axes in relation to the machine frame. The tool head includes a motor-driven tool, such as a milling cutter. In addition, a swivel unit is provided, which can be pivoted about a horizontal swivel axis relative to the machine frame; the swivel unit also includes a tool positioning device, via which a workpiece can be rotated about an axis of rotation which is oriented perpendicularly to the swivel axis.
Machine tools having such a configuration, sometimes also referred to as a modified Gantry design, offer excellent precision with regard to the form and positional tolerances that can be achieved on the workpiece under defined conditions. Nevertheless, unavoidable mechanical and/or thermal stresses of the machine tool may lead to deformations on individual components, which are not fully detectable by the position measuring systems integrated into the machine and are thus not fully compensatable. This causes errors in the workpiece to be machined, which means that it will not have the desired dimensions, and that the required form and positional tolerances will not be met.
For machine tools of a different type it is described in German Published Patent Application No. 10 2011 079 792 to address the aforementioned problems by integrating a measuring frame into the machine, which, if possible, cannot be deformed either by mechanical stressing or by thermal influences. In this particular case, the rectangular measuring frame is integrated into the stationary machine part and is made from a material having a low thermal expansion coefficient. Disposed on the measuring frame are components of position measuring systems, via which the position of the machining or spindle head in relation to the measuring frame is determined on the one hand, and the position of the workpiece in relation to the measuring frame is ascertained on the other. This makes it possible to decouple the position determination with regard to the tool and workpiece position from mechanical and thermal influences in the provided machine kinematics, and the aforementioned problems regarding the machine precision are able to be minimized. Because of the scale of a position measuring system which is situated on the underside of the workpiece table and has only a limited spatial extension, the travel range of the table detectable via this position measuring system is limited. Moreover, it is not explained how a suitably configured measuring frame can most usefully be integrated into a machine tool of the type discussed in the introduction that allows the relative positioning of workpiece and tool in five degrees of freedom in space.